Agriculture Reference
In-Depth Information
the years while the availability of other resources (such as water, energy, or light)
becomes more limited which, in turn, will increasingly hamper production (Melillo
and Gosz 1983; Bloom et al. 1985). Thus as Himalayan alder plantations age the
nitrogen and phosphorus use efficiency will decrease. The latter trends are con-
firmed by Sharma (1993). Binkley et al. (1992) have further reported that in its use
of nutrients, the red alder ( Alnus rubra ) is much less efficient than conifers. The
lower nutrient use efficiency of alder trees is however beneficial to other crops in
alder tree-crop associations such as cardamom because of the generally greater
availability of such nutrients related to the systems' faster nutrient cycling.
18.3.3 BiogeochemicalCycling
The nitrogen and phosphorus concentrations in the tissues of alder trees were
higher than those of forest mixed tree species (Sharma et al. 1994). This finding is
consistent with the higher concentrations of nutrients found in the red alder, com-
pared with conifers in mixed stands (Binkley 1983; Binkley et al. 1984). Both
nitrogen and phosphorus re-translocation from leaf before abscission was lower in
alder than mixed tree species. This was because of the higher availability and
uptake of these elements in the alder-cardamom system. The general concept of an
inverse relationship between availability and conservation is clearly applicable to
alder-cardamom and forest-cardamom agroforestry systems. The higher availability
of nitrogen and phosphorus for the alder trees resulted in lower re-translocation
which is indicative of its poor conservation strategy. However, this strategy is ben-
eficial for the associated crops in the alder tree plantations, i.e., cardamom.
The nutrient re-translocation of senescent alder leaves was positively related to
stand age for nitrogen, but negatively with phosphorus. The nitrogen re-transloca-
tion in young alder trees was minimal because it was sufficiently available through
fixation. However the demands for nitrogen increased with age while the contribu-
tion from fixation decreased, causing greater translocation. In the case of phosphorus,
its need for tree growth was high in younger stands where effective re-translocation
was recorded (Sharma et al. 2002b). Yet, its need and re-translocation decreased
with increasing stand age. The alder thus displayed contrasting physiological
behaviour for nitrogen and phosphorus at different ages; it was mostly governed by
the demand and availability of these nutrients.
The annual uptake and return of nitrogen to the soil in the alder-cardamom stand
was higher than the forest-cardamom stand, which can be attributed to nitrogen
fixation by the alder tree (Table 18.1). The rates of phosphorus uptake and return
through litter-fall and decomposition were also higher in alder-cardamom than the
forest-cardamom stand. This was probably a result of an increase in the rate of
phosphorus supply, attributable to geochemical and biological factors influenced by
the alder. Potential geochemical factors could be rhizosphere acidification (Gillespie
and Pope 1989) and biological factors could be rooting depth (Malcolm et al.
1985), soil enzyme activity (Ho 1979) and organic chelates (Ae et al. 1990).
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